These therapy balls can be used with kids to increase their core muscle strength and endurance for sitting. 12″ size is good for toddlers, but you may want 25″(65 cm) or 36″ (75 cm) for slightly older kids. It depends on if you want their feet to be able to touch the ground or not when they are sitting on the ball. They will be able to use their feet for stability if they touch the ground, but if their feet do not touch the ground they will need to use more of their core muscles (tummy and back muscles).